Test technology - Positioning

Precise workpiece positioning is essential for reproducibility of the measured values in measurement and test technology or also in gauge construction. Only when workpieces are precisely positioned and fixed can actual and target conditions be reliably compared to each other. This article highlights the challenges of positioning, introduces key components, and demonstrates how modern testing methods ensure accurate alignment.

Importance of precise position in metrology

For reproducible results in measurement and test technology or in gauge construction, it is imperative that the workpieces are precisely aligned and fixed during the test process. This creates an identical starting position, with the help of which the actual state can be reliably compared with the target state. Thoughtful positioning concepts are the key in fixture construction. For more information on gauge construction, please see our article Gauge design explained: Inspection fixtures for quality assurance.

Particularly for complex geometries, supporting surfaces must be designed in such a way that no unwanted tilting moments occur, which can distort the measurement results. In order to check form and position tolerances correctly, it is necessary to fix the workpiece in precisely defined reference systems - usually by means of a combination of a dowel pin (form-fit) and a contact surface (force-fit). Stress-free clamping is also crucial, since even minor deformations due to clamping forces can systematically distort the test result. To prevent this, defined clamping concepts are often used, which enable clear, reproducible, and tolerance-compensating workpiece support.

The 3-2-1 principle is a method in fixture design that is used to uniquely position a workpiece by limiting its six degrees of freedom (three translations and three rotations). It specifies fixation at 3 points in the main plane (or primary plane), 2 additional fixation points in the secondary plane, and one fixation point in the tertiary plane. For more information on position determination, see our article Locating workpieces in positioning devices.

Positioning challenges

Designers and inspectors face the same challenges over and over again: Environmental vibrations, thermal expansion, or complex geometries make it difficult to produce test fixtures with error-free, reproducible results. Even the smallest deviations in workpiece positioning can lead to significant measurement errors. Therefore, it is critical to take appropriate measures to minimize these influences. In addition to precise clamping systems and defined reference edges, the use of special damping materials also plays an important role. The influence of the operator, such as by manually inserting the workpieces, can also lead to unintentional positional errors if no standardized stops or guides are available. To minimize these influences, automated clamping systems and defined reference edges are particularly effective.

Components for precise positioning

A variety of mechanical, pneumatic, hydraulic and sensory elements are available for accurate and repeatable workpiece positioning, which are used as needed and depending on the test fixture:

• Locating pins
• Angles, angle plates and shims
• Stop bolts and blocks
• Linear guides and positioning units
• Proximity sensors, sensor rails, and sensor plates

Some of the positioning elements are described in detail below.

Locating pins

Positioning pins are used for the precise fixation of workpieces or components in fixtures and ensure repeatable placement. They are partly standardized, e.g. NAAMS and are available in various dimensions and versions as well as various materials. For a detailed look at the world of pin connections, see our article Pin connection - Connecting components with cylindrical pins - Procedure / Errors. Alternatively, fitting screws can also be used.

Combining a diamond-shaped locating pin with a round locating pin offers several advantages. While the position in direction A is absolutely fixed, the diamond shape of the second locating pin leaves some play in direction B. Thermal expansion and errors in the distance of the fastening holes can thus be compensated.

Linear guides

Linear guides enable precise, low-friction movement of workpieces or tools along defined axes and are essential for accurate positioning in machining processes. They consist of guide rails and matching carriages supported by rolling elements such as balls or rollers to ensure high load capacity and rigidity. In positioning devices, linear guides provide repeatable motion. Please also refer to our linear guide selection guide.

Proximity Sensors

Proximity sensors can be used to detect the presence or position of a workpiece without the need for direct contact. Inductive sensors detect metallic objects due to changes in the electromagnetic field, while capacitive sensors can also detect non-metallic materials. Proximity sensors check the correct position of workpieces, thereby avoiding incorrect positioning. For more information, see the article Proximity switch vs. limit switch - Differences and areas of application.

Adjustment tables

Adjustment tables or positioning tables form the backbone of a positioning device. They are mechanical units with guides, clamps and other attachment or feed mechanisms and serve as a stable platform for aligning and placing workpieces. To map the XY axis with the positioning tables, either two units or one positioning table for XY axes is required. If height adjustment is required, it is recommended to use a Z-axis table. Positioning tables are also available in X,Z directions and X,Y,Z directions.

For more information on positioning tables, please also read our article Manual Positioning Units - Precision in Small Space.

Clamping workpieces with clamping devices

Clamping devices can be used to securely clamp workpieces into the fixture. Clamping devices range from simple toggle clamps to more complex precision clamping devices such as magnetic angle fixators. The most suitable clamping device depends on the machining process. For example, three-jaw chucks, faceplates or collets are used during turning. When milling, it is recommended to use magnetic, hydraulic, or mechanical clamping devices (such as collets or vises). Three clamping devices are presented in detail below.

A magnetic angle fixator, also called a welding angle, is used to securely fix two magnetic metal parts at a defined angle, usually 90° or 45°, for example during welding or tacking. In addition to rigid models with fixed angles, there are also adjustable models for flexibly adjustable angle ranges. Depending on the application, variants are available with smooth surfaces for sheets or with recesses for pipes and round profiles. The strong magnetic force ensures reliable fixation, even without mechanical clamping devices.

A toggle clamp is a clamping tool that uses a lever mechanism to securely hold workpieces in place, creating a high clamping force (more details in our article). A toggle lever consists of several articulated levers and is used to transmit force according to the law of the lever. In order to protect the surface of the workpieces, pressure screws are used in most cases. MISUMI offers several tool clamps.

A angle plate is a precision ground metal tool used to fix a workpiece perpendicular to the base surface. Its right-angled surfaces and additional fastening options such as T-slots or clamping slots allow for stable and repeatable positioning. It is mainly used on milling, drilling and coordinate measuring machines, as well as marking-out tables. There it serves to precisely orient and securely clamp workpieces - whether for machining, measuring or preparation.

Clamp the workpiece correctly

Secure workpiece clamping is critical for reproducible and meaningful test results. Even the smallest deviations in position can cause measurement errors. Only stable fixation guarantees consistent test quality. The following considerations are important when selecting the clamping device:

• Define type of inspection: The requirements vary by test type, e.g. contactless, optical tests or tactile measurement methods. In non-contact testing, a low-reflection, exposed surface may be more important than clamping force, while tactile measurement processes rely on strong workpiece fixation. For more information on measurement procedures, see here.
• Ensure repeatability: Guides, stops, and mechanical reference points help to position the workpiece in the same position at all times.
• Reduce vibration effects: By using suitable damping materials, environmental vibrations can be shielded. More on this in the article Properties of damping materials and their use.
• Observe the compatibility of the measuring devices: The clamping system must not interfere with the measurement sequence.

Calibration and Positioning Check

Calibration in measurement technology is used to determine the measurement accuracy of a measuring device or measurement system by comparing it to a traceable reference standard. It is critical for reliable, reproducible measurements needed in quality assurance and manufacturing. In addition to calibration, the positioning check is also important: It is checked whether the workpiece is (repeatedly) precisely positioned within the device in the intended position. Even small deviations in the position can lead to measurement errors, even if the measuring device is calibrated correctly. Therefore, the combination of measuring system calibration and part positioning validation is essential for accurate and traceable measurement results.